Brake systems according to the prior art use a vacuum brake booster, i.e. they use vacuum as an auxiliary energy. Generally the vacuum produced in the intake pipe in an internal combustion engine or a mechanical vacuum pump coupled to an internal combustion engine serves as a vacuum generator. In particular in vehicles with an electric drive (electric vehicles or hybrid vehicles) electric vacuum pumps are also used.
From time to time, in modern vehicle designs (down sizing) the necessary minimum braking effect is no longer achieved when the vacuum supply or the vacuum brake booster fails, with the result that in this case a hydraulic brake boosting assistance function, which is implemented by means of the ESC (Electronic Stability Control) system, has to be activated. Such a brake boosting assistance function according to the prior art uses the signal of a vacuum sensor for this purpose.
In order to ensure the availability of the hydraulic assistance function and respectively generate a driver warning in the event of non-availability, it is necessary to monitor the vacuum sensor signal. Known methods for this are:                a) to use a redundant vacuum sensor,        b) to process a further vacuum signal, for example from the engine controller.        